Simply speaking some healthy bacteria produce bacteriocins - substances that can inhibit the growth of unwanted bacteria or even kill them. Some bacteria excrete their natural metabolites which inhibit the growth of other micoorganisms - like lactic acid bacteria which produce lactic acid and lower pH of the environment. There are more factors involved but these are basic I guess.

Also, I have heard that bacterial infections normally develop in eyes and mouth b/c the good bacteria count is low...so how can I keep it at healthy level?

1. If good bacteria have colonised a space then there is a good chance that there will be no room for bad bacteria to establish themselves.
2. To keep your eyes and mouth free of bad bacteria - avoid air borne projectiles [e.g. cow dung], poor quality food [e.g. crops grown with manure fertilisers], tobacco smoke, industrial fumes, alcohol, sexually-transmitted fluids, abstain from touching yourself [e.g. itching your eyes] and practice good oral hygiene.

Yup its true the colonised bacteria will compete for the food of the new bacteria, making it tough for the new bacteria to survive.

Also the old bacteria usually make biofilms, meaning they stick onto the surface as colonies working together to generate a film that will prevent other new bacterial colonies / species from colonizing the area.

Yup its true the colonised bacteria will compete for the food of the new bacteria, making it tough for the new bacteria to survive.

Also the old bacteria usually make biofilms, meaning they stick onto the surface as colonies working together to generate a film that will prevent other new bacterial colonies / species from colonizing the area.

The crucial thing is to have biofilms established by "good" bacteria, as usually happens when an infant is fed with mothers milk (plenty of bifidobacteria were found in intestined of infants and toddlers).

Another thing discovered is that "good" or rather probiotic bacteria (mainly lactic acid bacteria) ans some symbiotic bacteria somehow activate human immune response when they have contant with pathogenic bacteria.
It has something to do with TLR receptors.

Group of researchers led by PhD Yasmine Belkaid discovered that during infection DNA particles are released from symbiotic bacteria (maybe due to their lysis caused by pathogenic bacteria - thing yet unknown) and those DNA bind to TLR9 receptor and that trigger immune response. "Good" microorganisms may therefore act as an adjuvant.

This is interesting but many mechanisms are still to be discovered.

Jorge1907 - why do you thing the mechanisms I described are odd? It was proved that metabolsm products like lactic acid inhibit the growth of some bacteria, so do bacteriocins. The latter substances are ecologically important - they seem to play role in bacterial competition in the environment (especially soil) where bacteria compete for nutrients and try to outgrow other bacteria.

Are good bacteria essential for body...if there were no bad bacteria...would I still need them...can't my body carry out its functions(a little less efficiently) without them? B/c it is a little freaky having those things on me....(talking theoretically)

Odd in the context that these have been described and have not been shown to be more than isolated phenomenona. From organic acids produced by staph, to bacteriocins, to your DNA particles to nathan's crowding and biofilm (and not touching himself) - etc. etc. application beyond the isolated example is speculation. Remember there are more microbial cells in/on pur body than human cells and human genome found alot of what appeared to be microbial sequences

Drop "good" - it has no technical meaning. View normal flora and the skin - staph, malassezia some anaerobes diphtheroids and for most males resident dermatophyte. Specific strains are maintained on the individual and often on specific parts of the body. To the extent that they discourage newcomers - mechanisms will likely be different and will involve directly or indirectly the host.

In another context - specific vaginal flora makeup is not only maintained by the individual, it's also consistent among groups - such that there are 7 distinct vaginal floras in the world. Blazer says he can follow ancient routes of immigration by typing the Helicobacter in folks stomachs.

If we were to look at "good" in the context of probiotic - I've observed consumption of a probiotic bifido strain to establish changes - loss of a DNA signal in gut flora - before the bifido signal was seen.

Even in the environmental sense, the mechanisms are unclear. Strains from the environment - supplemented in the lab with new genetic capabilities and returned to that environment - do not move back in.

Hey jorge....I am really interested in what you have to say...can you write in a language I can understand please?

Also, what do I need to avoid to not get bad bacteria?(I mean I thought plants are healthy for you...but how if fungus not?....I mean common sense wouldn't tell you that water is bad...but it is....thats why humidifer needs to be cleaned.)

One more thing....do these bacteria colonies also stop virus? Since w/ them...count don't matter coz they disguise themselves....can they only enter through blood stream? If thats true, then whenever have bruises....isn't odds against us and how can we stop being infected and fight it w/o medicine?

Sure. Bottom line - except for some isolated observations, we really don't understand how normal flora is maintained and how late arrivers are rejected. In fact, we barely understand normal flora. Viruses? Not that I'm aware but who knows.

To avoid? Wash your hands. In modern society with adequate nutrition, that's about all you can/need to do unless you're immunocompromised.